Datasheet

1191
SAM3X / SAM3A [DATASHEET]
Atmel-11057C-ATARM-SAM3X-SAM3A-Datasheet_23-Mar-15
40.7.3 Time Management Unit
The CAN Controller integrates a free-running 16-bit internal timer. The counter is driven by the bit clock of the CAN
bus line. It is enabled when the CAN controller is enabled (CANEN set in the CAN_MR register). It is automatically
cleared in the following cases:
 after a reset
 when the CAN controller is in Low-power Mode is enabled (LPM bit set in the CAN_MR and SLEEP bit set in
the CAN_SR)
 after a reset of the CAN controller (CANEN bit in the CAN_MR register)
 in Time-triggered Mode, when a message is accepted by the last mailbox (rising edge of the MRDY signal in
the CAN_MSR
last_mailbox_number
register).
The application can also reset the internal timer by setting TIMRST in the CAN_TCR register. The current value of
the internal timer is always accessible by reading the CAN_TIM register.
When the timer rolls-over from FFFFh to 0000h, TOVF (Timer Overflow) signal in the CAN_SR register is set.
TOVF bit in the CAN_SR register is cleared by reading the CAN_SR register. Depending on the corresponding
interrupt mask in the CAN_IMR register, an interrupt is generated while TOVF is set.
In a CAN network, some CAN devices may have a larger counter. In this case, the application can also decide to
freeze the internal counter when the timer reaches FFFFh and to wait for a restart condition from another device.
This feature is enabled by setting TIMFRZ in the CAN_MR register. The CAN_TIM register is frozen to the FFFFh
value. A clear condition described above restarts the timer. A timer overflow (TOVF) interrupt is triggered.
To monitor the CAN bus activity, the CAN_TIM register is copied to the CAN _TIMESTP register after each start of
frame or end of frame and a TSTP interrupt is triggered. If TEOF bit in the CAN_MR register is set, the value is
captured at each End Of Frame, else it is captured at each Start Of Frame. Depending on the corresponding mask
in the CAN_IMR register, an interrupt is generated while TSTP is set in the CAN_SR. TSTP bit is cleared by
reading the CAN_SR register.
The time management unit can operate in one of the two following modes:
 Timestamping mode: The value of the internal timer is captured at each Start Of Frame or each End Of
Frame
 Time Triggered mode: A mailbox transfer operation is triggered when the internal timer reaches the mailbox
trigger.
Timestamping Mode is enabled by clearing TTM field in the CAN_MR register. Time Triggered Mode is enabled by
setting TTM field in the CAN_MR register.